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According to most sources of information I have found (A Quora answer and books), when galaxies become quasars, they destroy all life in their host galaxy, as they output so much radiation that they can disintegrate DNA and do lots of bad stuff, like destroying the ozone layer.

But according to some information I have found, such as this site, The Milky Way itself was a quasar or atleast, an active galaxy 6,000,000 years ago, when massive gas streams were blasted from Sagittarius A*, the Milky Way's supermassive black hole. This explains the enormous Fermi Bubbles found lingering above our galaxy, and the abnormal amount of gamma rays being found in that region. Quasars are known to be so powerful that they make GRBs seem like a insignificant firework.

The proof that the Milky Way was a quasar is the Fermi Bubbles, giant bubbles of gas found lingering over our galaxy. According to studies, the Fermi Bubbles emit a unusually high amount of gamma rays. This is way too much radiation to be produced by dark-matter annihilation. Studies suggest that the reason for the Fermi Bubbles being so radioactive and energetic is due to the fact that the Fermi Bubbles may have been the remnants of a quasar event, when too much gas fell into Sgr A* and was blasted out in jets, which formed the Fermi Bubbles. This also explains the reason why they are emitting so much gamma rays, due to the rapid ionization of so much gas by this quasar event. According to this Iopscience source, temperatures in the Fermi Bubbles may range from $10^6K$ to $10^8K$. Also, it has been shown that the "cool" gas in Northern Fermi Bubbles, have been clocked at $2⋅10^6K$. This may have been a spectacular sight in the prehistoric night sky, but terrible news for life, due to intense irradiation of hard gamma-rays and X-rays. enter image description here.

But at that time, the Earth already had advanced life on it, and even apes were present at the time. If the Milky Way was a quasar, then it should have outputted so much radiation into the environment that it would have destroyed the ozone layer. Also, the radiation would have sterilized the planet to a lifeless rock, and it would have taken several millions, or even billions of years for Earth to recover from such a mass extinction. And yet, everything on Earth seems normal. The ozone layer is intact, humans haven't mutated into horrendous creatures, nor is our planet fried to a crisp. Nothing is out of place.

I cannot get my head around this, quasars are supposed to destroy all life in the host galaxy, and not spare life. It should have turned Earth into a barren wasteland and destroy even the hardiest microbes, and destroy the ozone layer, even stripping the Earth of its own atmosphere, if it was powerful enough.

How could have life survived on Earth, when the Milky Way was a quasar 6 million years ago?

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    $\begingroup$ Hard to answer a straw man argument. Clearly the Milky Way "AGN" was not powerful enough to disrupt life on Earth. Your question would benefit from cited (reputable) sources that say Earth should have been turned "into a barren wasteland". $\endgroup$
    – ProfRob
    Oct 8, 2022 at 7:50
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    $\begingroup$ " when galaxies become quasars, they destroy all life in their host galaxy" they don't, and whatever source you got that from is garbage. $\endgroup$
    – eps
    Oct 9, 2022 at 0:20
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    $\begingroup$ Given that we currently know of exactly one galaxy with life in it (and only on one planet in one star system at that), a claim that all life is necessarily destroyed some other galaxy is mere speculation. $\endgroup$ Oct 10, 2022 at 13:53
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    $\begingroup$ Humans (any Homo species, not just Homo sapiens) started evolving about 3.3 - 2.8 million years ago. Obviously we had ancestors that were alive 6 million years ago, but we wouldn't describe them as human or even proto-human yet. en.wikipedia.org/wiki/Human_evolution $\endgroup$
    – CJ Dennis
    Oct 10, 2022 at 22:37
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    $\begingroup$ "...humans haven't mutated into horrendous creatures..." Citation needed. $\endgroup$
    – James
    Jan 25, 2023 at 20:00

7 Answers 7

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An active galactic nucleus doesn't emit energy equally in all directions. It may form "jets", and if you are looking into the AGN at the right angle, and then nucleus is active enough, then you see a quasar.

Being too close to the jets of a quasar would make complex life hard on a planet. But there are three things that may have protected Earth. We are not too close, being in a spiral arm of the galaxy. We were not in the direct line of any jets. And the nucleus might not have been especially active.

So it simply isn't the case that an AGN will necessarily sterilize the galaxy. The Milky Way AGN was never strong, close, and positioned well enough to destroy life. And we know this simply because we are here.

Your internet link is about being "hit by a quasar" which is rather unclear, it certainly doesn't talk about merely sharing a galaxy with an AGN.

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    $\begingroup$ in my reading being "hit by" is pretty clear in meaning definitely more than being in the same galaxy. $\endgroup$
    – Hobbamok
    Oct 10, 2022 at 8:06
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How much dangerous a quasar at Sag A* could be? A Fermi-inspired estimation:

Assume it is 1000x the luminosity of the Milky way. This is rather at the high end (as per Wiki) and rounding out its intrinsic anisotropy.

We do see some light from the Milky way. We have hard time seeing Sag A* because it is rather obscured by the galaxy plane-bound gas and dust. But let's assume it is not obscured. Now, imagine 1000 times the Milky way light in a single point-like object.

Pretty bright object in the night sky (maybe even visible during the day) and still WAY dimmer than a full moon.

Half of it being hard gamma-rays? Our atmosphere is pretty much opaque to these. The thermosphere may swell as in the periods of high solar activity.

Sounds rather survivable.

The other factors that are unaccounted for (like the anisotropy of the quasars and the galaxy plane light extinction) work in our favor as well.

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I think the source of confusion here is that the two sources you mention implicitly have different definitions of what quasar is. Firstly, to our current understanding, quasars are not a well-defined type of objects with common traits. This is an observational phenomenon which we can see mostly in the early universe, and it is caused by very bright active galactic nuclei that have a particular orientation towards us. However, the active galactic nuclei come in all sizes, depending on how much matter is falling into the central black hole.

So, the first article describes the habitability of the galaxy with a very active nucleus, on par with the brightest we can see, which does not mean that when the Milky way nucleus was active, it reached the same level of power output.

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A lot of answers here talking about jets, but that only applies to perhaps the very high energy (gamma ray) emission. Most of the luminous quasars we see are not seen via their jets but are extremely powerful and variable sources of ultra-violet and optical emission. The former would be problematic for life on Earth.

Fortunately, our Galactic centre, from where the UV emission would arise, is behind a wall of dust that is almost entirely opaque to UV radiation. Extinction at UV wavelengths (in terms of astronomical magnitudes) is 2-3 times higher at UV wavelengths than in the optical and amounts to 60-90 magnitudes for the Galactic centre.

i.e. Any UV flux from the Galactic centre is reduced by a factor of at least $10^{60/2.5}=10^{24}$, rendering any quasar-related UV emission harmless.

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Quasars emit their energy along a pair of antipodal jets. Those jets have an angular width of no more than about 2.5 milliarcseconds, which is about $5.5\times 10^{-7}$ degrees, or $10^{-8}$ radians, roughly. In other words, the jets are extremely narrow. The slant height is very nearly equal to the height of the cone. The core of the Milky way is perhaps at most 10,000 light years thick. The volume of one of the jets after 10,000 light years is approximately $(\pi/3) \times 10^{-4}$ cubic lightyears. The density of the Milky Way's core is about .4 stars per cubic light year. Thus one jet is expected to include roughly $4\times 10^{-5}$ stars in it. Well less than one whole star expected, even if we include both jets. We'd need to look at about 12,500 such quasars (not all galaxies with a quasar will have the same specs as the Milky Way, but the story is much the same regardless) before we'd expect to find even a single star within any one of the jets.

As such, a quasar is expected to hit nothing but diffuse dust anywhere near itself and its host galaxy; and the amount of nearby dust it's going to hit is probably going to have a total mass several orders of magnitude lower than that of our sun.

So when anyone is talking about a quasar "hitting" something, and they aren't talking about dust, they are by default talking about it hitting something in some other galaxy entirely. It's like throwing a dart and comparing hitting the bullseye versus hitting something within the same city neighborhood as the bullseye. One requires obscenely more precision than the other.

If the quasar originated at a distance equal to that of the Andromeda galaxy, and the Earth was within the cone of one of the jets, then assuming I calculated correctly the Earth would receive about $.7\times 10^{-15}$ of the total energy emitted by the jet. The strongest quasars emit on the order of $10^{41}$ Watts, and the average is about an order lower at $10^{40}$ Watts. Meaning we'd receive from such a nearby quasar about $35\times 10^{24}$ Watts at the extreme end, and about $3.5\times 10^{24}$ Watts for an "average" quasar. The Sun, by contrast, has a total energy emission of about $4\times 10^{26}$ Watts. The Earth itself only receives about $1.7\times 10^{18}$ Watts. So a quasar hitting Earth from the distance of Andromeda would swamp out the Sun in relative energy input—I haven't accounted for any form of energy lost, such as from hitting dust along the way, but my guess is it's not significant enough at this scale—, which would be decidedly devastating. Just about any quasar within the local cluster of galaxies, pointed at the Earth, would be deadly to all life. Thankfully quasars appear to be a feature of the early universe only: we only see such quasars at high redshifts, very far away. As such all quasars are evidently too far away and too unlikely to be pointed directly at us to pose any actual threat to Earth at any point in its existence. Your article is probably generating confusion, as indicated in vvotan's answer, as it seems to be conflating "active galactic nucleus" with "quasar"; and that's conflating rectangles with squares. Just as all squares are rectangles but not all rectangles are squares, a quasar is usually considered an extreme and particular form of an AGN, but not all AGN's are quasars.

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You misinterpreted the article.

the Milky Way recently (by cosmic standards) went through a quasar stage in its evolution.

That does seem to indicate that the Milky Way turned into a quasar, but in fact some words are missing.

The summary is a better description of what happened:

Bottom line: Astronomers at the Harvard-Smithsonian Center for Astrophysics believe that our galaxy’s central black hole might have been in a quasar phase of activity six million years ago.

Thus, not the whole galaxy, just the very center.

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    $\begingroup$ "just the very centre" is what a quasar is. $\endgroup$
    – James K
    Oct 9, 2022 at 19:22
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  1. Saggitarius A was a AGN 6 million years ago, so it is not necessary for it to become a quasar, it can also be an seyfert galaxy with less luminosity, in fact ESO says "All quasars are AGNs, but not all AGNs are quasars"..

  2. It is not necessary for it to be a blazar or a quasar pointed towards the Earth and in fact I think that most of the quasars aren't blazar because there is a lot of space other than Earth so there are more chances of it colliding with other celestial object

  3. Even quasars and seyferts follow the inverse square law and redshift even though it wouldn't cause much effect

So all these points above restrict for an Ordovician-Sillurian type extinction caused by a Quasar

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